The magnetic resonance imaging (MRI) system is a medical image diagnostic device configured to apply a high frequency magnetic field and a magnetic field gradient to a subject laid in a static magnetic field, measure the signal generated from the subject through nuclear magnetic resonance, and form an image of the signal.
The magnetic field gradient intended to generate the one-dimensional magnetic field intensity distribution in a space around the subject is used for imparting the location information to the signal. Generally, the magnetic field gradient generator includes a coil and a power source for driving the coil. As it is necessary to impart three-dimensional location information, three coil groups and the power sources are prepared so that the magnetic field gradient is applied separately to the three orthogonal axes (x, y, z).
The power source with significantly high output is necessary for generating the magnetic field gradient required for photographing. For this, there is a driving method using combined power sources each with low output in parallel with one another (for example, refer to JP-A-9-94244). This method is configured to divide the coil of the respective axes into a plurality of subcoils so that the coils of the respective axes may be driven by a plurality of power sources. Accordingly, it is possible to generate sufficient magnetic field gradient even if the low-output power sources are separately employed.
Use of a plurality of power sources for driving may cause the problem of output timing misalignment among the respective power sources. Variance in characteristics of the power sources and the difference in impedance among subcoils may be the main cause of the timing misalignment. The timing misalignment further leads to the artifact and distortion in the photographed image. Adjustment has to be made to completely synchronize the respective power sources.
The adjustment method for synchronization employs the delay circuit for shifting the time for input waveform of the power source. One of the methods is configured to allow the ammeter to measure the current waveform flowing through the subcoil so that the differential waveform among the current waveforms becomes zero. Another method is configured to adjust the static magnetic field intensity at the center in the bore so as to prevent change in the intensity while monitoring the static magnetic field intensity (for example, refer to Japanese Unexamined Patent Application Publication No. Hei 9-94244).